Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

module/icp/core/kcf_prov_tabs.c

@@ -377,7 +377,7 @@ kcf_provider_zero_refcnt(kcf_provider_desc_t *desc)
 			mutex_exit(&desc->pd_lock);
 			break;
 		}
-		/* FALLTHRU */
+		fallthrough;
 
 	case CRYPTO_HW_PROVIDER:
 	case CRYPTO_LOGICAL_PROVIDER:

module/icp/io/aes.c

@@ -976,7 +976,7 @@ aes_encrypt_atomic(crypto_provider_handle_t provider,
 	case AES_GMAC_MECH_INFO_TYPE:
 		if (plaintext->cd_length != 0)
 			return (CRYPTO_ARGUMENTS_BAD);
-		/* FALLTHRU */
+		fallthrough;
 	case AES_GCM_MECH_INFO_TYPE:
 		length_needed = plaintext->cd_length + aes_ctx.ac_tag_len;
 		break;